Recording materials which utilize an electron-donating colorless dye and an electron-accepting compound are disclosed, for example, in Japanese Patent Publication Nos. 14039/70 and 4160/68. The minimum desirable properties which these types of recording materials must possess are: (1) to have satisfactorily high color density of a developed image and sufficiently high coloring speed; (2) to generate no fog (no color development upon storage prior to use); and (3) to produce a developed image which retains sufficient fastness after color development. However, recording materials which satisfy all of these reguirements have not yet been obtained.
In particular, earnest studies on property (1) described above have been made in recent years concomitant with advances in the technology of recording systems. Various measures to provide recording materials with property (1) have been adopted with respect to heat-sensitive recording materials, which form one group among recording materials. For instance, one measure consists of the use of an electron-accepting compound having a melting point of 60.degree. C. to 100.degree. C. In another measure, an organic acid and a phenolic compounds are used in combination as an electron-accepting substance. Further, the use of a polyvalent metal salt of a compound having an alcoholic hydroxyl group, the use of a copolymer prepared from hydroxyethyl cellulose and maleic anhydride, the addition of waxes, the addition of a carboxylic acid ester (e.g., dimethyl isophthalate, diphenyl phthalate, dimethyl terephthalate or the like) are a sensitizer, and the addition of hindered phenols have all been examined for efficiency in hightening both color density of a developed image and coloring speed.
However, such measures are insufficient to impart heat-sensitive recording materials with property (1) so as to meet recent technological advances relating to high-speed recording.
On the other hand, methods for enhancing the stability of a developed image have also been investigated extensively. For instance, the method of adding a phenol derivative, such as 2,2'-methylenebis(4-methyl-6-tert-butylphenol) or the like, is described in Japanese Patent Publication No. 43386/76. Moreover, the method of adding a water-insoluble modified (e.g., rosin modified) phenol resin is disclosed in Japanese Patent Application (OPI) No. 17347/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and the method of adding an terephthalic acid ester is described in Japanese Patent Application (OPI) No. 72996/81.
However, all heat-sensitive recording materials produced in accordance with the above-described methods result in insufficient stability of the developed image.
Further, it is well-known to use various kinds of fluoran derivatives as the precursors of dyes for pressure-sensitive or heat-sensitive manifold paper, as disclosed in U.S. Pat. No. 3,825,432, West German Patent Application (OLS) No. 2,262,127, and Japanese Patent Publication No. 38245/76. All color images obtained from these known fluoran derivatives do not have true black color tone, and some of the fluoran derivatives suffer from the defect that they are unstable in the atmosphere, while others have the defect that dyes produced therefrom are inferior with respect to fastness to light, moisture, etc.
Moreover, various compounds have been examined for usefulness as colorless dyes for recording materials which can provide a pure black hue. Nevertheless, only three kinds of compounds, namely, 2-anilino-3-alkyl-6-aminofluoran, 2-anilino-3-chloro-6-aminofluoran and spiro[acridineisobenzofuran]-10-aryl-3'-one, have been found to be colorless dyes capable of providing a black hue, and the number of compounds having such a property is very limited. These compounds also have the defect similar to that of the abovementioned compounds.